Optical communication system incorporating automatic dispersion compensation modules

ABSTRACT

In accordance with the invention, an optical communication system is provided with one or more automatic dispersion compensation modules. Each module has an adjustable dispersion element, a data integrity monitor and a feedback network whereby the monitor adjusts the dispersion element to optimize system performance. In a preferred embodiment the dispersion compensating modules comprise chirped fiber Bragg gratings in which the chirp is induced in the grating by passing a current along distributed thin film heaters deposited along the length of the fiber. The magnitude of the applied current determines the dispersion of the grating. A data integrity monitor is configured to sense the integrity of transmitted data and to provide electrical feedback for controlling the current applied to the grating.

FIELD OF INVENTION

The present invention relates to optical communication systems and, inparticular, to an optical communication system incorporating one or moreautomatic chromatic dispersion compensation modules for optimizingsystem performance.

BACKGROUND OF INVENTION

Optical fiber communication systems are beginning to achieve their greatpotential for the rapid transmission of vast amounts of information. Inessence, optical fiber system comprises a source of information-carryingoptical signals, an optical fiber transmission line for carrying theoptical signals and a receiver for detecting the optical signals anddemodulating the information they carry. Optical amplifiers aretypically located along the line at regular intervals, and add/dropnodes are disposed at suitable locations for adding and dropping signalchannels.

Optical communication systems are usually based on high purity silicaoptical fiber as the transmission medium. Conventional systems aretypically designed to transmit optical signals in a wavelength rangewhere longer wavelength components are subject to slightly longerpropagation time delay than shorter wavelengths. This chromaticdispersion did not deteriorate the information content of the opticalsignals because early systems used a single channel at a wavelengthwhere dispersion is low.

As it has become desirable to utilize many channels over a wider rangeof optical wavelengths (WDM systems), group velocity dispersion hasrequired more precise compensation. WDM systems are becomingincreasingly important for their ability to transmit vast amounts ofinformation and for their ability to incorporate network functions suchas add/drop and cross connecting. But as the number of channelsincreases in WDM systems, dispersion compensation becomes increasinglyimportant.

Dispersion gives rise to undesirable pulse distortion, which can limitbandwidth and/or transmission distances. Dispersion compensation is thuscritical to the performance and ultimate commercial success ofcommunication systems and particularly of those that operate at 10Gbit/s per wavelength channel and higher. Typically, dispersioncompensation is accomplished using especially designed optical fiberspossessing specified dispersions or with chirped fiber gratings, both ofwhich work by providing a fixed amount of dispersion of an equal signand opposite magnitude to that of a given fiber span. Both of thesetechnologies have been demonstrated and are being incorporated intolightwave systems.

The performance of high speed WDM lightwave networks will dependcritically on the details of the system design and particularly on thelevel of in-line dispersion and dispersion slope compensation as well asnonlinear effects occurring in the dispersion compensating fiber (DCF).In such systems small variations in optical power, due for example toimperfect gain flattening of optical amplifiers, can result inadditional nonlinear phase shifts that can modify the optimal dispersionmap of the system. This problem is exacerbated by a reduced dispersionbudget associated with imperfect dispersion slope compensation over awide bandwidth of operation. For example, in a typical system operatingwith approximately 40 nm of bandwidth, and with an uncompensateddispersion slope of 0.05 ps/nm² km, the accumulated divergence in thedispersion (assuming approximately 60% compensation in DCF) isapproximately 1.2 ps/nm km. The corresponding dispersion budget istypically taken as twice this value, giving 2.4 ps/nm km. It followsthat the maximum transmission distance (L) that can be achieved beforeincurring a significant penalty is given by

L<104,000/(B ² |D|)(Gb/s)²ps/nm  (1)

where B is the channel rate and D the dispersion of the fiber, is 32 kmfor a 40 Gbit/s system and 512 km for a 10 Gbit/s system. Therefore, insystems that operate at 10 Gbit/s, 40 Gbit/s or higher, the combinationof dynamic fluctuations in the optical power and the reduced dispersionbudget associated with the dispersion slope will impose significantchallenges in designing networks.

Because of these variations with wavelength and level fluctuations, itis very difficult, with fixed or static dispersion compensating devices,such as DCF or conventional chirped Bragg gratings, to manageeffectively the unavoidable dispersion and nonlinearities in a highspeed WDM lightwave network. Optical networks typically have slowvariations in the optimal path dispersion that arise whenever there is achange in the total optical power and/or power distribution in thenetwork.

FIG. 1 shows an exemplary conventional optical fiber transmissionsystem. The system comprises a transmitter 10, a transmission fiber path11 and a receiver 12. The system may also include one or more staticdispersion compensation modules 14. The transmission path typicallycomprises conventional optical transmission fiber, and optionallycomprises one or more optical amplifiers 13 (typically erbiumamplifiers). At the transmitter output the pulse shape is sharp and welldefined. After passage through, typically hundreds of kilometers ofoptical fiber 11, including a multiplicity of optical amplifiers 13, thepulse has broadened, and is considerably distorted. After transmissionthrough a (static) dispersion compensation module 14 the pulse isreshaped and is mostly restored into its original shape.

In realistic optical systems, significant nonlinearities occur in thetransmission fiber and in other components of the network. Thesenonlinearities prevent complete dispersion compensation with a staticmodule. With a fixed dispersion compensating element, optimization isdifficult because nonlinearities give rise to additional phase shift andpulse distortion that are dependent on optical power and thereforechange with time as the traffic on the network changes or the gain ofthe erbium-doped amplifiers change. It can therefore be very difficultto ensure complete restoration of the pulse at all points and at timesin any realistic high-bit optical network. This results in pulsedistortion that is not compensated for, can accumulate and cansignificantly degrade system performance.

In addition, in optical systems comprising more than one channel(Wavelength Division Multiplexed systems) the incurred pulse distortioncan be channel dependent and, as before, can vary with time. In such asystem, one requires the ability to add or to drop specific channels atselected nodes in the network. This adding and dropping can beaccomplished, for example, with fiber gratings that reflect the desiredchannel and transmit all other channels. In performing add/dropping, theaverage power in the fiber can fluctuate. Gain flatteners are capable ofaccommodating for such changes, but small fluctuations in the opticalpower are unavoidable. As a consequence of this power fluctuation, thepulse can undergo more pulse distortion in the transmission fiber andthus can require a different and time-varying amount of dispersioncompensation.

Several approaches to eliminate the effects of unwanted dispersion havebeen proposed: (a) optical pulse regeneration (b) optical phaseconjunction (OPC); (c) optical solitons. All of these techniques arepromising and have been demonstrated in laboratory environments, butthey have important limitations for application to realistic systemssuch as those being deployed now. For example, OPC (also referred to asmid-span spectral inversion) requires that the dispersion compensationmust be performed at the mid-point of the link. This requirement isoften not possible to satisfy with many optical network designs. Solitonsystems are very attractive, but they require precise management of thedispersion map of the system and suffer other disadvantages.

The use of dynamic dispersion elements in communication systems has beenproposed to compensate for a change in the fiber dispersion resultingfrom a network reconfiguration (see J. X. Cai et al., Proceedings ofOptical Fiber Conference, 1998, page 365 (1998)). The reference proposesthe use of a dispersion monitor and a dynamic dispersion element tocompensate for transmission pathlength changes. The monitor measures thetotal chromatic dispersion between the transmitter and the receiver.However, this approach measures only the dispersion and cannot directlymeasure the integrity of transmitted data. This is a critical flawbecause the data is also distorted during transmission through nonlineareffects, where accumulated phase shifts not measured by a dispersionmonitor, vary the dispersion compensation required to achieve optimumperformance. Thus there is a need for an improved automatic dispersioncompensation module to optimize system performance.

SUMMARY OF INVENTION

In accordance with the invention, an optical communication system isprovided with one or more automatic dispersion compensation modules.Each module has an adjustable dispersion element, a data integritymonitor and a feedback network whereby the monitor adjusts thedispersion element to optimize system performance. In a preferredembodiment the dispersion compensating modules comprise chirped fiberBragg gratings in which the chirp is induced in the grating by passing acurrent along distributed thin film heaters deposited along the lengthof the fiber. The magnitude of the applied current determines thedispersion of the grating. A data integrity monitor is configured tosense the integrity of transmitted data and to provide electricalfeedback for controlling the current applied to the grating.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, advantages and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in connection with the accompanyingdrawings. In the drawings:

FIG. 1 schematically depicts a conventional optical fiber communicationsystem using static dispersion compensation modules;

FIG. 2 schematically depicts an optical fiber communication systemcomprising one or more automatic dispersion compensation modulesaccording to the invention;

FIG. 3 shows an exemplary adjustable dispersion compensating element foruse in the module of FIG. 2;

FIGS. 4(a)-(d), 5(a)-(b), 6(a)-(b), and 7-10 are graphical illustrationsuseful in understanding the operation of the adjustable dispersioncompensating element of FIG. 3; and

FIGS. 11-13 schematically depict exemplary data integrity monitors foruse in the module of FIG. 2.

It is to be understood that these drawings are for purposes ofillustrating the concepts of the invention and, except for the graphs,are not to scale.

DETAILED DESCRIPTION

This description is divided into three parts. Part I describes anoptical communication system employing an automatic dispersioncompensating module. Part II describes exemplary adjustable dispersioncompensating gratings for the module, and Part III describes exemplarydata integrity monitors for the module.

I. Communication System With Automatic Dispersion Compensation Module

Referring to the drawings, FIG. 2 shows schematically an exemplaryoptical communication system including one or more automatic dispersioncompensation modules 20 according to the invention. The system issimilar to that in FIG. 1 except for the addition of the automaticdispersion compensation module 20. The module can supplement or replaceone or more static compensation modules (14 of FIG. 1) in order tooptimize system performance.

Each automatic dispersion compensation module 20 comprises an adjustabledispersion compensator 30, a data integrity monitor 31 and a feedbacknetwork 32 whereby the monitor 31 adjusts the dispersion compensator 30to optimize system performance. In this exemplary module, the adjustabledispersion compensator 30 comprises an optical circulator 32 coupled tothe transmission fiber and an adjustable dispersion compensating grating(DCG) 33. The monitor 31 can comprise an optical receiver coupled to thetransmission line and a data processor for deriving a feedback controlsignal from the received signal.

In operation, a portion of the signal on the transmission fiber 11 issampled as by tap 34. The signal is analyzed to provide a measure of theintegrity of transmitted data, and the measure controls a feedbacksignal to control the adjustable DCG 33. If redundant coded data istransmitted, the analysis can be as simple as a threshold level chosento be highly sensitive to errors in the system. Feedback is chosen tominimize the error rate. The redundant coding can be error detectionsignals already used in optical transmission such as the bit-interleavedparity 8 error detection codes in the B-octets of the Sonet protocol.

In the adjustable dispersion compensator 30, signal light from thetransmission fiber 11 enters one port of the circulator 32 and isdirected to an adjustable DCG 33 at a second port. The adjustable DCG 33in this example is an adjustable Bragg grating operating in reflectionmode. The compensated signal light is reflected back to the circulator32 from which it is directed to a subsequent segment of the transmissionfiber 11.

The adjustable DCG 33 can be any one of a variety of adjustabledispersion grating devices including gratings having chirp adjustable bytapered resistive heaters, by tapered strain relief, or by magneticallyadjustable strain. Resistive heater adjustable dispersion gratingdevices are described in Eggleton et al., U.S. patent application Ser.No. 08/183,048 entitled “Optical Grating Devices With Adjustable Chirp”filed Oct. 30, 1998, now U.S. Pat. No. 5,487,436. Magneticallyadjustable devices are described in S. Jin et al., U.S. application Ser.No. 09/159,178 entitled “Tunable Dispersion Compensator and OpticalSystem Comprising Same filed Sep. 23, 1998, now U.S. Pat. No. 6,148,127,both of which are incorporated herein by reference.

The data integrity monitor 31 can be any one of several types ofmonitors capable of sensing the level of system performance inmaintaining the quality of transmitted data. Such performance can besensed indirectly, as by analysis of the transmitted spectrum, ordirectly as by analysis of error rates.

II. Adjustable Dispersion Compensating Gratings

FIG. 3 illustrates an exemplary adjustable DCG 33 useful in theembodiment of FIG. 2. The grating 40 comprises a sequence of indexperturbations 41 in a fiber 42 spaced to form an apodized Bragg grating.The grating 40 is disposed in thermal contact with an electricallycontrollable heat-transducing body 43, which can be a heat-generatingbody or a body which actively removes heat. The body 43 is typically aheat-generating body such as a resistive film on the fiber. The body 43can linearly vary in resistance along the grating 40 to provide linearlyvarying heating of the grating and thus produce a linear chirp. Thisvariation in resistance can be achieved, for example, by varying thethickness of the body 43 along the grating. A pair of electrodes 44, 45provides electrical contact with wires 46, 47 from an electrical source48, such as a source contained in the monitor feedback circuit.Advantageously, the grating 40 is enclosed in a cylindrical tube about 1cm in diameter for thermal isolation. The structure, fabrication andoperation of such thermally adjustable grating is described in greaterdetail in Eggleton et al, U.S. patent application 08/183,048, filed Oct.30, 1998, now U.S. Pat. No. 5,487,436, and entitled Optical GratingDevices With Adjustable Chirp which is incorporated herein by reference.

The following is a specific example of the design and analysis of aspecific adjustable grating 40.

EXAMPLE

We present detailed results on a fiber grating device that providesconstant dispersion over its bandwidth and that can be dynamicallyadjusted by varying an applied voltage. This device relies on a lineartemperature gradient induced along the length of the grating byresistive heating in a metal coating whose thickness varies inverselywith position along the length of the fiber. The chirp rate, and thusthe dispersion, is controlled by varying the applied current. Numericalmodeling and experimental evidence confirms that, to a very goodapproximation, the temperature varies linearly along the length of thegrating and the resulting chirp is linear. We demonstrate experimentallycontinuous tuning of the dispersion from 300 ps/nm to 1350 ps/nm, withless than 1 W of electrical power. Measurements of the gratingdispersion characteristics reveal a group delay ripple with an averagedeviation from linearity of approximately 10 ps, indicating that thedevice would be well suited to operation in 10 Gbit/s lightwave systems.In the following we describe the principle of operation of this device,present a simple model for prescribing the chirp as well as moredetailed numerical simulations of heat flow through such structures, andfinally summarize some optical measurements of the devices.

The device consists of an unchirped, apodized fiber Bragg grating thatis coated with a thin film of metal whose thickness varies along thelength of the grating. Current flowing through the film generates localresistive heating in a manner that is determined by the thickness of thefilm. Control over the film thickness, therefore, allows considerablecontrol over the temperature profile and, in turn, the chirp of thegrating. If we assume, for simplicity, that (i) the temperaturedistribution in the core of the fiber follows the distribution ofheating power produced by the resistive film (i.e. the flow of heatalong the length of the fiber does not seem to cause the shape of thetemperature distribution to deviate strongly from the distribution ofheating), (ii) the increase in temperature is linearly related to theheating power (i.e. the flow of heat out of the fiber is approximatelylinear even though radiation and convection are strictly non-linearprocesses), and (iii) shifts in the Bragg resonance are linearly relatedto changes in temperature, then it is possible to derive a simpleexpression that relates the chirp to the thickness profile. We first useof the fact that the local resistance, R(z), is inversely proportionalto the thickness, t(z):

R(z)˜1/t(z)  (2)

where z is the position along the grating. Given that the local powerdissipated is given by,

P(z)=I ² R(z),  (3)

and that the local temperature change is proportional to the powerdissipated then we can write,

Δλ_(B)(z)˜ΔT(z)˜I ² /t(z)  (4)

where Δλ_(B)(z) is the local shift in the Bragg wavelength, ΔT(z) is thechange in temperature, and I is the applied current. This equationrepresents a simple, approximate description of the behavior of thesedevices, and provides guidance for engineering tunable grating devicesand the associated dispersion. For example, if we assume a thicknessthat varies inversely with distance along the length of the grating(i.e. t(z)˜I/z) then Δλ(z)˜z corresponding to a grating in which theBragg wavelength varies linear along its length. To a good approximationthe dispersion of the grating is given by D=dΔτ/dΔλ, where Δτ=2nL/c isthe round trip time of the grating, n is the refractive index of thefiber, L is the length of the grating and c is the speed of the light invacuum, and Δλ is the grating's bandwidth. Thus, using Eq. (4), andassuming the t(z)˜1/z film profile, it is straightforward to show thatthe dispersion of the grating scales with the inverse square of theapplied voltage or current, i.e. D˜1/V²˜1/I².

To treat the general case, without simplifying assumptions, we usednon-linear finite element modeling to compute the steady state thermaldistributions in operating devices with geometries like those describedabove. We assumed cylindrical symmetry, and solved the equation forthermal diffusion:

∇·(κ(r)∇(T(r,z)))=0  (5)

where T(r,z) is the temperature and κ(r) is the thermal conductivity.The calculations were performed for a two-material structure consistingof a glass fiber (diameter=120 μm) coated with a tapered film of silver.Perfect thermal contact between the silver and the glass was assumed andcontinuity of normal heat flux normal was enforced at the boundarybetween the two materials. The calculations used adaptive meshrefinement and radiative and convective heat loss from the surface ofthe silver at rates given by σE(T⁴−T_(o) ⁴), andA(T−T_(o))^({fraction (5/4)}) respectively, where T is the temperatureof the surface of the metal, T_(o) is the temperature of thesurroundings, σ is the Stefan-Boltzmann constant, E is the emissivity ofthe surface of the metal, and A is a constant that characterizes naturalconvection in air. The thickness of the metal coating varied between 5and 20 microns and the size of simulated system along the length of thefiber was chosen large enough for the temperature distribution in thecoated region to be insensitive to the boundary conditions at the endsof the fiber (temperature fixed to that of the surroundings).

FIGS. 4a-4 d are useful in understanding the heating of the grating.FIG. 4a illustrates a device with a film whose thickness isapproximately inversely proportional to distance along the fiber. (Forthe purposes of simulation, we divided the coating into three linearsegments to approximate a continuous coating whose thickness dependsinversely on position.) FIG. 4b shows the computed temperature at thecore of the device. The results for this particular system indicate thatthe simplifying assumptions described in the previous paragraph arevalid at locations away from the ends of the coating (i.e. equation (4)is applicable at these locations). The dependence of the temperature onposition at the ends of the coating provides a rough measure for thelimits of validity of assumption (i) outlined in the previous paragraph.(See FIGS. 4(c) and 4(d). It reveals the extent to which thermaldiffusion along the length of the fiber “smears out” the temperaturedistribution expected based solely on the geometry of the thin filmheater. As FIGS. 4c and 4 d show, for this system, the effective axialthermal diffusion length is ˜1 mm. This length is, however, a sensitivefunction of the precise rate of thermal transport out of and along thestructure; direct experimental measurements will be required to obtainan accurate estimate of this quantity.

Measurements were performed on devices that used gratings that were 8 cmin total length. The gratings were suitably apodized to reduceinterference effects due to the sharp boundaries that are present in atthe ends of uniform Bragg gratings. They were fabricated in standardtelecommunications fiber (containing germanium) which was appropriatelyhydrogenated.

FIG. 5(a) shows optical measurements of the thickness of the film alongthe length of the fiber grating. For this particular sample, thethickness of the metal film varies from approximately 5 μm to 50 μm. Thesolid line is the target profile, which varies inversely with distancealong the length of the fiber, and was designed to achieve a desiredtemperature excursion between the ends of the grating, for a prescribedcurrent. The uncertainty in the measurement of the film thickness is ofthe order of 1 μm. FIG. 5(b) shows the deviation of the film thicknessfrom the solid line shown in FIG. 5(a). For most of the length along thefiber grating, the deviation is comparable to the measurementuncertainty (1 μm) and exceeds this value only near the end of thegrating.

FIG. 6(a) shows typical measured reflection spectra of the fiber gratingdevice for increasing values of applied voltage: (i) 0V, correspondingto the grating in its unchirped state; (ii) 0.611V; (iii) 0.82V and(iii) 1.1V. The uniform broadening in the reflection spectrum arisesfrom heating that varies monotonically and approximately linearly alongthe length of the grating. Note that there is also an overall shift inthe center of the reflection peak that accompanies the broadening. FIG.6(b) shows the change in the width of the reflection peak as a functionof the shift in the center wavelength of the reflection peak; thelinearity of this data is consistent with the linear temperaturedependence of the device. In general, the shift is undesirable and thuswould need to be compensated by mechanical strain or an additionalsource or sink of heat.

FIGS. 7-10 illustrate the dispersive properties of the device. FIG. 7shows the measured group delay of the device versus wavelength for anapplied voltage of 1 V. The dashed line shows a linear fit to the dataand has a slope of D=−353 ps/nm. Note that there is small structure inthe group delay, shown clearly in FIG. 8, which shows the deviation inthe measured group delay from linearity. The peak-to-peak fluctuation isless than 10 ps. The resolution of this measurement was 0.005 nm andthus it is unlikely that any structure exists on a finer scale. Thisstructure, which is typical in chirped Bragg gratings similar inmagnitude to the best results reported elsewhere and provides a measureof the quality of the gratings and of the applied chirp.

The possible causes for the structure in the group are as follows: (a)imperfections in the grating fabrication process that can result innoise in the local effective index in the grating profile; (b)fluctuations in the fiber core diameter, which can give rise to smallvariations in the effective index of core mode and thus provideadditional “noise” in the grating profile; (c) non-ideal apodization,which can result in undesired interference effects in the grating,although this would appear as systematic ripple and thus is probablyinsignificant; and (d) deviations in the film thickness profile from thedesired one, or nonuniformities in the resistivity or surface texturethat give rise to undesired variations in temperature. Although it isdifficult at this point to determine definitively the dominant cause offluctuations, FIG. 6(b) provides some indication that variations in filmthickness may be important. It is worth noting that, as a percentage,the deviation in the measured coating thickness from strictly inversedependence on length is somewhat larger than the deviation in themeasured group delay from linear. We speculate that thermal diffusionalong the length of the fiber tends to “smooth out” slightnon-uniformities in the distribution of heating power that occur onshort length scales, and that this effect reduces the impact of smallvariations in thicknesses or other film characteristics.

FIG. 9 shows the measured group delay response of the grating device fordifferent values of applied voltage: (i) 0.53V, (ii) 0.611V, (iii)0.72V, (iv) 0.82V, (v) 0.94 and (vi) 1.1. The figure illustrates thetunability of the dispersion. Note that the “noise” in the measuredgroup delay slightly increases for the larger dispersion values. Evenfor the largest dispersion value obtained, the deviation from linearityis only 20 ps.

FIG. 10 shows the measured group velocity dispersion as a function ofapplied voltage. The solid line is a theoretical fit to the dataassuming that the dispersion scales with the inverse of the square ofthe applied voltage. The maximum dispersion measured was approximatelyD=−1350 ps/nm, corresponding to an applied voltage of 0.4V, and acorresponding electrical power of 0.4 W. Increasing the applied voltagehas the effect of increasing the temperature gradient and thus thebandwidth of the grating, which reduces the dispersion of the grating.

The maximum applied voltage in this experiment was approximately 1.1V(approximately 1 W electrical power), which corresponds to a peaktemperature close to 200 C. We note that this is well below the level atwhich reliability can become an issue. In particular, operating at hightemperatures can have many practical implications, such as requiringhigher initial index changes to offset the annealing, which is requiredto ensure the stability of the device.

This work demonstrates a design for a fiber Bragg grating device thatuses distributed on-fiber resistive heaters to achieve desiredtemperature distributions in the fiber and thus dispersion. The deviceshave attractive features that include power efficient operation, compactsize, simple fabrication, and controllable optical properties. Wedemonstrated experimentally continuous tuning of the dispersion from−300 ps/nm to −1350 ps/nm, with an average deviation from linearity ofapproximately 10 ps. Previous research has shown that for optimumoperation of a dispersion compensating grating, the average deviationfrom linearity in the measured dispersion, needs to be substantiallyless than the bit period, preferably of the order of 10%. The devicepresented here, with a measured group delay deviation from linearity of10 ps, is thus well suited to operation in a 10 Gbit/s lightwave systems(bit period of approximately 100 ps). For operation at 40 Gbit/s,improvements in the design of the grating and metal coating arenecessary in order to reduce the ripple even further. We note that themaximum achievable dispersion in the grating device is limited only bythe absolute length of the grating and thus could be increaseddramatically by using long fiber Bragg gratings.

As mentioned previously, we speculate that limited axial thermaldiffusion, which tends to “smooth out” the effects of slight variationsin heating power that arise from small unwanted changes in coatingthickness or other imperfections that occur over short length scales (˜1mm), tends to improve the linearity of the group delay.

III. Data Integrity Monitors

FIG. 11 illustrates an exemplary data integrity monitor 20 useful in theembodiment of FIG. 2 comprising an optical filter 120, anoptical-to-electrical converter (O-E converter) 121 and an RF spectrumprocessor 122. A data processor 123 is provided for calculating thefeedback control signal to the compensator. The filter 120 can be achannel filter, and the O-E converter can be a photodiode. In operation,the filter selects a spectral region to be analyzed. The O-E converter121 converts the filtered optical signal into a corresponding electricalsignal. Spectrum processor 122 determines the RF spectrum of the O-Econverted signal, and the data processor 123 analyzes the spectrum andcalculates a generates a feedback signal for optimizing the system. Asimple scheme for spectrum optimization involves minimizing the lowfrequency components. A description of how to generate such anoptimizing feedback signal can be found in F. Heismann et al.,“Automatic Compensation of First Order Polarization Mode Dispersion in a10 Gb/s Transmission System”, Proceedings ECOC, ′98, pp. 529-530 (1998).

The advantage of this particular monitor is that it does not require areceiver or calculation of the error rate of the received data. Thefeedback control signal can be generated without extracting theelectrical data and clock of the optical signal. An optional orsupplementary feature is to provide a remotely located dispersioncompensator 30′ which can be remotely controlled based on the feedbacksignal.

FIG. 12 shows an alternative embodiment of a monitor 20 using forwarderror detection. Here feedback signal generation can be built into areceiver terminal. The monitor comprises an optical filter 120, an O-Econverter 121, a data-clock recovery circuit 130, and an error detectioncircuit 131, all of which are typically found in a receiver. It furthercomprises a data processor 132 for calculating the error-rate andgenerating an optimizing feedback control signal based thereon. Thefeedback control signal is to optimize data integrity through thesystem, i.e. to minimize errors.

In operation, the optical data of each wavelength channels carryoverhead bits for a forward error correction algorithm. Typically thisalgorithm will be implemented as a Reed Solomon forward error correctioncode (see for example BAG. Lee, M. Kang and J. Lee, “Broadbandtelecommunication technology”, Artic House, 1993 on forward errorcorrection algorithms including the Reed Solomon code and theirimplementation). The optical wavelength channels pass through thevariable dispersion compensator, which can be co-located with thereceiver terminal, or placed remotely from the terminal. In the lattercase, the control signal to the dispersion compensator is transmitted tothe remote dispersion compensator 30′ via a management signaling system.At the terminal, the signal is filtered optically to select a specificwavelength channel. The data are converted into the electrical domain inthe O-E converter and the clock and data are extracted from the receivedwavelength channel. The recovered data are passed to the errorcorrection circuit, where, based on the implementation of the errorcorrection algorithm, data are corrected and passed to the output of thereceiver terminal. The error correction circuit also providesinformation on the number of bits that were corrected in a certain timeperiod. Based on this number, the bit-error rate—before errorcorrection—can be calculated corresponding to the present value of thecontrol signal applied to the dispersion compensator. By comparing thepresent error-rate and control signal with the error-rate and controlsignal corresponding to a previous setting of the control signal, theratio D_(error rate)/D_(Control signal) can be derived. This ratio tellshow much the control signal should be changed to minimize the bit errorrate, and in what direction. The calculated control signal is applied tothe dispersion compensator, and a new bit error rate is measured andused to calculate a new control signal.

FIG. 13 shows another embodiment of a monitor 20 which does not requireerror correction algorithms but rather uses the error detectioncapability of the framing format used to carry the data. This monitordiffers from that of FIG. 12 primarily in the programming of the dataprocessor 132. One example of such framing format is SONET which provideerror detection (but not correction) via the B octets in the SONETframe. Thus, the error rate can be calculated by accessing andprocessing the B octets in the SONET frame. The implementation of thefeedback signal generation is similar to the implementation describedfor FIG. 12. This implementation of the error rate detection andfeedback signal generation works for any signaling protocol where thedata are packaged in frames and the frames contains error detection (oreven error correction) information. Examples of such transmissionprotocols include ATM where the error rate can be obtained by processingthe HEC byte and FDDI, where the presence of transmission errors isindicated in the frame status section of the FDDI frame. Thus the errordetection circuit can detect errors in an ATM format or in an FDDIformat.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be made by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:
 1. An improved optical fiber communication systemcomprising an optical transmitter for providing at least one opticalsignal channel, an optical fiber transmission path, and an opticalreceiver, said system subject to variation in the optimum pathdispersion as a function of optical signal power, the improvementcomprising: at least one automatic dispersion compensation modulecoupled to said transmission path, said module comprising an adjustabledispersion compensating element, a data integrity monitor for monitoringcomprising an error detection circuit for detecting errors in the datatransmitted on the system, and a feedback circuit from said monitor tothe adjustable dispersion compensating element for adjusting theadjustable dispersion compensating element to optimize data integritythrough the system.
 2. The system of claim 1 wherein said adjustabledispersion compensating element comprises a Bragg grating withadjustable chirp.
 3. The system of claim 1 wherein said adjustabledispersion compensating element comprises a Bragg grating with thermallyadjustable chirp.
 4. The system of claim 1 wherein said adjustabledispersion compensating element comprises a Bragg grating withmagnetically adjustable chirp.
 5. The system of claim 1 wherein saiddata integrity monitor comprises, a data processor for calculating anoptimizing feedback signal based on said errors.
 6. The system of claim1 wherein said error detection circuit detects errors in the framingformat used to carry data.
 7. The system of claim 6 wherein said errordetection circuit detects errors in a SONET format.
 8. The system ofclaim 6 wherein said error detection circuit detects errors in an ATMformat.
 9. The system of claim 6 wherein said error detection circuitdetects errors in an FDDI format.
 10. The system of claim 1 wherein saiderror detection circuit comprises a forward error correction circuit.11. The system of claim 10 wherein said error correction circuitutilizes a Reed Solomon forward error correction algorithm.
 12. Thesystem of claim 1 wherein said optical transmitter comprises amultiwavelength optical transmitter for providing a plurality ofwavelength-distinct optical signals channels, and said receivercomprises a multiwavelength optical receiver.